1. Field of the Invention
The present invention relates to an improvement in flexible risers connected to an articulated structure emplaced in deep water for underwater operations, particularly for petroleum extraction.
2. Description of the Prior Art
In Fr. Pat. No. 2,307,949 assigned to the assignee of this application, a riser for an articulated structure for deep-water petroleum extraction is described. Such riser is eccentrically disposed with respect to the articulated structure and is connected to a head pipe by a joint which is equivalent to an aligned fitting, and is supported against the interior wall of the well guide. The result is that the zone where the maximum stresses are applied when the articulated structure oscillates is localized between the sliding piston (which may be considered as a translationally mobile aligned fitting) and the fitting on the head pipe. A device for areal application of stresses to the riser enables buckling stresses to be reduced.
While fatigue is manifested solely on the lower element, it has been found that with the substantial eccentricity of the arrangement there can be substantial deformation of that element, with serious consequences for the behavior of said element. To alleviate these problems, in Fr. Pat. No. 2,513,305 also assigned to the assignee of the present application, a mechanical guiding device is proposed for said lower element, which device employs an articulated parallelogram system having the effect of subjecting said lower element to the same stresses as those to which a riser disposed along the axis of the articulated structure would be subjected.
Although this mechanical device does have advantages, greater attention needs to be given to continuous functioning of the areal stressing means. The slightest error in operation of these stressing means gives rise to a hazard of buckling, over the riser as a whole and particularly in its lower element--to the extent that a rupture can become inevitable.